"The Little Brother and Sister" is a well-known European fairy tale which was, among others, written down by the Brothers Grimm in their collection of Children's and Household Tales (Grimm's Fairy Tales). It is alternatively known as "Little Sister and Little Brother" or (in the Grimm's version) "Brüderchen und Schwesterchen".
Tired of the cruel mistreatment they endure from their wicked stepmother, who is also a witch, a brother and sister run away from home one day. They wander off into the countryside and spend the night in the woods. By morning the boy is thirsty, and so the children go looking for a spring of clear water. But their stepmother has already discovered their escape, and has bewitched all the springs in the forest. The boy is about to drink from one, when his sister hears how its rushing sound says "Whoever drinks from me will become a tiger".